1. Field of the Invention
The present invention relates to an access network system for access to a backbone network such as Internet, and in particular to a Point-to-Point Protocol (PPP) termination device thereof.
2. Description of the Related Art
PPP is a data link protocol that is very popular for data communication between two computer systems. Typically, the PPP protocol is used for computer dial-up access to an Internet Service Provider (ISP) over telephone lines. PPP also provides Authentication, Accounting, Service Management System (SMS), bandwidth allocation for each subscriber, etc.
In the case of Internet access, it is necessary to terminate PPP to transfer IP (Internet Protocol) packets to the Internet. Conventionally, a dedicated device is installed at the gate to the ISP to perform PPP termination.
FIG. 1 shows an example of a conventional access network system. In the access network system, a personal computer (PC) 1 is connected to an ADSL Transceiver Unit-Remote (ATU-R) 2, which is connected to a Digital Subscriber Line Access Multiplexer (DSLAM) 3 through ADSL/VDSL line 1101, wherein ADSL stands for Asymmetric Digital Subscriber Line and VDLS for Very high-speed DSL. The DSLAM 3 is connected to an ATM switch (SW) 4 through ATM OC-3c transmission service 2101. The ATM SW 4 is connected to PPP termination device 5 through ATM OC-12c transmission service 3101. In this example, the PPP termination device 5 is the dedicated device for terminating PPP 501 to transfer IP packets to a backbone network 6 such as Internet. Therefore, the PPP termination device 5 is substantially a gate to the ISP.
A packet communication system based on the PPP protocol has been disclosed in Japanese Patent Application Unexamined Publication No, 10-322399. In this packet communication system, IP packets are converted to PPP packets according to the PPP protocol and then the PPP packets are in turn converted to ATM cells and vice versa. Such an arrangement allows a plurality of ATM logical lines to be multiplexed into a single physical line without loss of PPP functions and ATM additional functions.
However, according to the prior art, as shown in FIG. 1, it is necessary to provide the PPP termination device 5 at the gate of the backbone network 6, where a large number of subscriber's signals are multiplexed. It is also necessary to increase the number of PPP termination devices depending on an increasing number of subscribers.
Alternatively, it can be considered that the ATM layer and the higher AAL5 layer processing and PPP termination are implemented in the DSLAM 3. However, the virtual Channel (VC) itself must be processed at a layer higher than the AAL5 layer, resulting in more complicated structure of the DSLAM 3, increased system cost, and complicated system management.